Method of removing gold, silver, palladium, or the like



Jan. 2, 1940. s, R, MAM 2,185,858

METHOD OF REMOVING GOLD, SILVER, PALLADIUM, OR THE LIKE Filed June 27,1956 M/Vf/VTO/F I 6: R. MAS 011/ aygawm Patented Jan. 2, 1940 METHOD OFREMOVING GOLD, SILVER, PALLADIUM, OR THE LIKE *Sfumner R. Mason,Wilmette, 111., assignor ."to Western Electric Company, Incorporated,.New'

York, N. Y., a corporation ofNew'Yoi-k Application June 27, 1936, sci-a1No. 81,693

3 Claims.

This invention relates 'to a method of removing gold, silver, palladium,or the like, from a metal base.

- An object of the invention is to provide an 5 effective and eflicientmethod of removing gold,

silver, palladium, or the like, from a metal base.

In accordance with one embodiment of the invention, a duo, copper, brassor bronze base to be stripped of gold is made the anode in anelectrolyte comprising a solution of concentrated sulphuric acid inwhich the base is both chemically and electrolytically inert, while thegold is readily removed thereby from the base.

A complete understanding of the invention may be had by reference to thefollowing description taken in conjunction with the accompanyingdrawing, in which:

Fig. i is a plan view of a tank for electrolytically treating gold,silver or palladium coated zinciferous articles to remove the coatingtherefrom;

Fig. 2 is a side elevation 01 a rack for holding the articles whilebeing treated, and

Fig. 3 is an end view of the rack shown in Fig. 2.

It is a well known fact that gold is difllcult .to dissolve in simpleacids, while metals such as copper, zinc, brass, or bronze, arerelatively readily soluble. In view of this fact, considerabledifiiculty has been experienced in the past in removing gold coatingsfrom articles of these metals. For instance, in the manufacture of goldplated brass base transmitter electrode cups, the gold plating on thecups is frequently defective. When the gold is removed by mechanicalmeans, a considerable amount of brass must be removed therewith in orderto remove all of the gold, and the reduction of the size of theelectrode cup renders it useless for its intended purpose and thusresults in a considerable economic loss.

By means of the present invention, the gold may be completely removedfrom the transmitter cup without affecting the dimensions of the brassbase which may be then replated with gold for further use. In removingthe gold from the transmitter electrode cups, the cups may be mounted onthe rack 6 of a conducting material which has a plurality of screws 1secured thereto, the alternate screws extending in opposite directions,whereby the large number of electrode cups may be mounted on the rack.The rack 8 may be provided with a hook 8 by means of which a pluralityof racks may be hung over a rod 9 in a tank containing a sulphuric acidsolution having a specific gravity in'the vicinity of 1.65. The positivelead of a direct current source of electrical energy may be connected tothe rodl, and the negative terminal is connected to a plurality ofcathodes II which comprise a number I of rods of nickel or othersuitable cathode material. When an electric current is passed betweenthe electrodes with the electrode 9 as an anode, the gold coating on thearticles is removed. While gold is not soluble in sulphuric acid, itappears 10 that immediately adjacent the anode some of the sulphuricacid is converted into persulphuric acid which dissolves the gold fromthe transmitter cups, but as soon as the dissolved gold passes from theimmediate vicinity of the anode, it is 18 precipitated out fromthesulphuric acid and collects in the bottom of the tank as a substantiallypure gold sludge. When the current is first applied to the electrode, acurrent of rather high density flows which decreases with time andeventually when all of the gold has been removed the current almostceases to flow. While it is not desired to be limited to any particulartheory of operation, it appears that when the sulphuric acid has asuiliciently high concentration of about 2 60%, a protective oxide filmis formed over the brass which renders the article completely passive inthe operation, with the result that the dimensions of the article arenot affected, and the article may be again plated to be reused.

If the bath is in operation for a long period, it may heat up and resultin an attack on the brass. It is advisable, therefore, to providecooling means for a bath that is to operate for long periods to preventthe temperature from rising substantially above F.

The invention has been described particularly in connection with theremoval of gold from zinc, copper, brass or bronze. The method, however,is equally applicable to the removal of palladium, silver and othermetals from such a base, although other methods are well known forremoving some metals other than gold from a base of these metals. In theremoval of a gold coating irom'zinc, copper, brass, or bronze, the basemetal 45 is not attacked in the method described, but gold may also beremoved from other metals, such as iron, in cases where a slight attackon the base metal may be tolerated.

It has been found that small amounts of chromium trioxide'may be addedto the sulphuric acid solution to accelerate the process, and in freshlymade electrolytes of sulphuric acid the addition of small amounts oinickel sulphate accelerates theprooess to some extent. Concentratedhydrochloric acid may also be substituted for the sulphuric acid,although an appreciable attack on the brass will take place, which insome instances may not be objectionable.

While specific embodiments of the invention have been described, it willbe evident that many changes and modiflc: tions may be made therein,without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of removing gold coating from a copper composition base,which comprises anodically electrolyzing the coated base in an aqueoussolution having suillcient sulphuric acid to produce a solution having aspecific gravity of 1.65, thereby precipitating the coating metal.

2. A method of removing gold coating from a copper composition base,which comprises placing the coated base in an aqueous sulphuric acid'solution having sufflcient sulphuric acid to produce a solution having aspecific gravity of 1.65 and containing a small amount of chromiumtrioxide, and passing a current through the solu-

